1. Field of the Invention
This invention relates to a method for removing gaseous inclusions or bubbles from viscous liquids produced during the generation and/or processing of the viscous liquids. Exemplary of such viscous liquids is molten glass.
2. Description of Related Art
A number of commercially important materials are produced by processes involving viscous liquids in which gaseous inclusions or bubbles are generated in the viscous liquids during the production process. Quality specifications for the final product typically require the removal of gaseous inclusions over a certain size and may also require the removal of gaseous inclusions to a specified number per unit volume of end product. Typical of such materials is glass, which is produced in a high temperature process in which the raw batch materials used to produce the glass are melted to produce a highly viscous molten liquid. Because the gaseous inclusions cannot be removed from the solid end product, they must be removed while the precursor material is still in the viscous liquid state.
Under normal gravity, a gaseous inclusion will rise to the surface of a liquid. This is a consequence of the lower density of an insoluble gas. Increasingly precise mathematical descriptions have been developed that describe this well known phenomenon in various liquids over a wide range of liquid viscosities, e.g. Stokes Law:Vαd2·g·(ρ1−ρg)/μ
Stokes Law states that the velocity at which a bubble rises is proportional to the square of the bubble diameter, proportional to the force of gravity, proportional to the difference in density between the liquid and the gas, and inversely proportional to the viscosity of the liquid. Thus, gaseous inclusions in viscous liquids rise very slowly through the liquids to the surface where they escape from the liquids, and the speed at which the gaseous inclusions rise through viscous liquids increases as the viscosities of the liquids increase. However, providing sufficient time for the gaseous inclusion to evolve from the viscous liquids is often impractical or undesirable. For processes such as glass melting which are carried out at high temperatures, holding the molten glass at temperature until the gaseous inclusions evolve can result in substantial additional costs and limits the methods that are practical for removal of the inclusions.
Most methods for speeding the removal of bubbles from liquids, particularly viscous liquids, take advantage of Stokes Law. Proposed and implemented methods for bubble removal include 1) pulling a vacuum on the liquid to increase bubble diameter and bubble velocity; 2) spinning the liquid to increase the gravitational constant g and, thus, increase bubble velocity; 3) heating the viscous liquid with local heating by various means, such as electrodes, burners, microwaves and the like, to decrease the liquid viscosity which, in turn, leads to higher bubble velocity; 4) injecting additional bubbles by using bubblers, adding a “fining agent”, or injecting a light gas such as helium into the viscous liquid, producing bubble coalescence which effectively increases bubble diameter and, thus, bubble velocity; 5) passing the liquid over a “shelf” to create a thin layer which reduces bubble removal time simply because bubbles have less distance to travel to reach the upper surface of the liquid; 6) using acoustic or ultrasonic energy to cause the bubbles to vibrate, or to coalesce, or to be pushed towards coalescence zones or the surface, thereby enhancing the removal of the bubbles from the viscous liquid; and 7) stirring the liquid by mechanically lifting the liquid from the bottom toward the surface or by heating to create convective currents to carry the bubbles toward the surface, thereby reducing the time required for the bubbles to rise to the surface.
U.S. Pat. No. 3,244,496 to Apple et al. teaches a glass fining method and apparatus for removal of bubbles from the molten glass in which a screen or perforated sheet of platinum is located below the glass surface as a means for providing bubble nucleation in the glass. Bubbles initiating on the screen adhere to the screen and grow to the point at which several bubbles join to form a larger bubble such that buoyancy causes it to neck down and break free, causing them to accelerate to the glass surface and break. U.S. Pat. No. 3,261,677 to Plumat teaches a method for enhancing the removal of bubbles from molten glass in which the molten glass is introduced into a refining chamber in which it is spread out in a wide and thin layer so as to prevent the formation of an appreciable temperature gradient. As a result, the glass remains exposed to the very hot atmosphere of the glass melting chamber with which the fining chamber is attached, thereby maintaining its highly fluid nature. This is said, in turn, to assist the easy rise and escape of the bubbles. U.S. Pat. No. 4,406,683 to Demarest, Jr. teaches a method and apparatus for removal of gas inclusions from a molten glass pool in which a gas-inclusion-permeable, refractory metallic or ceramic screen is inserted in the downstream flowing, upper portion of the pool to diminish the glass flow adjacent the surface by viscous drag forces while permitting gas inclusions to rise through the screen to the surface of the molten glass pool to dissipate into the atmosphere.
However, each of these methods has limits with respect to capital cost, energy cost, and practicality. Vacuum systems are costly to build and complex in terms of operation. Centrifuges can be complex and are impractical when working with high temperature liquids such as molten glass. Heating the viscous liquid to lower the viscosity, whether using burners, electrodes, or microwaves costs energy. The addition of new bubbles can lead to complexity, can add cost for the gas, and does not assure complete capture of the smallest bubbles that are the most difficult to remove. Thin film bubble removal is impractical because a large surface must be maintained without variations in temperature or flow rate and without excessive wear of the “shelf”. Acoustic or ultrasonic approaches, although promising, suffer from difficulties in scaling to a size that will effectively work with the large volumes of liquids commonly processed on an industrial scale. And, finally, stirring methods are of limited utility and must be implemented with care to avoid the addition of new bubbles into the liquid.